The midfielder was stretchered from the field with protection around his ankle, with Sergio Aguero forced off minutes earlier, on an otherwise positive night at the Etihad Stadium
Having been beaten by Liverpool before dropping points at home to Sunderland in a surprise 2-2 draw, City could ill-afford another slip-up at the Etihad Stadium.
|VIEW FROM THE ETIHAD STADIUM
|By Paul Macdonald
Manchester City's flying start led to a comfortable victory that keeps a degree of pressure on Liverpool in the title run-in, and the influence of David Silva and Sergio Aguero over this team was in evidence once again.
Both limped off, and Silva's personal concern at this stage will not only be for City, but for his Spain aspirations at the World Cup. His crestfallen expression as he lay on the stretcher does not bode well for a quick return - this routine win has came with a significant cost.
As for West Brom, it was over before it began. The gameplan disappeared without trace from the second minute when Zabaleta headed home, and Mel's naive side must hope that teams below them perform worse - it's not clear where the wins will come from.
David Silva's late withdrawal on a stretcher provided the only concern for City, who made a strong start and took the lead three minutes in when Zabaleta's acrobatic header evaded the attempted clearance of Craig Dawson.
The right-back was quick to react after Ben Foster had parried a shot from Aguero and the striker added a second goal seven minutes later.
Graham Dorrans produced a stunning half-volley to keep the visitors in the game during a frantic opening, but Demichelis restored his side's two-goal lead before the interval.
The win keeps City in with a fighting chance of claiming the title, while West Brom remain two places and three points above the relegation zone having played one game fewer than the three sides immediately beneath them.
City, who brought Silva, Gael Clichy and Edin Dzeko into their starting XI, made an ideal start when Zabaleta followed up Aguero's saved effort to head home and settle any nerves at the Etihad.
Aguero spooned another effort over the bar on only his second start after returning from injury, but soon shook off any rustiness to score his first goal since January, a low shot from outside the area beating Foster as the goalkeeper dived to his left.
West Brom then came close through Stephane Sessegnon's powerful drive before City went from the sublime to the ridiculous and gave their visitors the opportunity to pull a goal back.
A shambolic free-kick routine saw City concede possession and, when West Brom broke, Matej Vydra slipped in Dorrans, who juggled the ball superbly past Clichy and volleyed home.
Despite struggling to find the fluency they had demonstrated early on, City scored again nine minutes before the break.
Demichelis joined countrymen Zabaleta and Aguero in getting on the scoresheet, finding the net from close range after Vincent Kompany had flicked a corner into his path.
Kompany made a fine block to deny Sessegnon before half-time, but the second period provided much less in the way of excitement.
In stark contrast to Wednesday's draw with Sunderland, there was little late drama for City, who appeared comfortable as the game moved towards a conclusion.
However, their evening was soured when Silva was taken off injured with his right leg in a brace following an innocuous challenge with Billy Jones.